The present invention is an improvement upon the method and system disclosed in commonly owned copending patent application Ser. No. 863,303 filed Dec. 22, 1977 entitled "MIXING OF SECAM COLOR-T.V. SIGNALS," the entire disclosure of which is incorporated herein by reference.
The method and system of that application relate to the mixing of SECAM color-T.V. signals. The mixing method of that application is briefly reviewed here as follows: A plurality of original SECAM color-T.V. signals are to be mixed. Prior to mixing, each original SECAM color-T.V. signal is split into its luminance component and its color component. The color component is then frequency-demodulated, to yield a video-frequency color signal. The color signal has the form of two color-difference signals, time-division-multiplexed in alternate respective horizontal line intervals, i.e., the first color-difference signal during the first horizontal line interval, the second color-difference signal during the second horizontal line interval, the first color-difference signal again during the third horizontal line interval, and so forth. This color signal, comprised of two time-division-multiplexed color-difference signals, is then used to amplitude-modulate a carrier, and the thusly amplitude-modulated carrier is superimposed upon the associated luminance signal component, to form a frequency multiplex of the luminance and color signals. Such a frequency multiplex is formed from each one of the original SECAM color-T.V. signals to be mixed. Each such frequency multiplex is fed to a mixer via a single respective channel; the mixer is accordingly classified as a one-channel mixer, in the sense that each original signal need not be furnished using two input channels. The output signal of the mixer is then frequency-separated into a mixed luminance component and a mixed color component, the latter still being in amplitude-modulated form. The amplitude-modulated mixed color component is then amplitude-demodulated, to yield a video-frequency mixed color signal in which a first mixed color-difference signal and a second mixed color-difference signal are time-division-multiplexed in alternate respective horizontal line intervals. The mixed color signal is then used to frequency-modulate a carrier and the thusly modulated carrier is then superimposed upon the mixed luminance signal in accordance with SECAM standards, to yield the mixed SECAM color-T.V. signal desired.
With the mixing technique disclosed in that application, the carrier used for the suppressed-carrier amplitude-modulation of the individual original color signals must be fed to the synchronous amplitude-demodulator employed to amplitude-demodulate the mixed color signal at the output of the mixer. The carrier is for example fed to the synchronous amplitude-demodulator via a cable. Due to differing cable lengths feeding to the mixer, differences in the circuit stages within the mixer, and/or due to differing phase settings in the amplitude modulators and in the synchronous demodulator, and other causes, a phase error develops as between the pre- and post-mixed color-difference signals or, equivalently, the carrier before and after the mixing. In order to eliminate these phase errors, adjustable-phase stages can be inserted between the individual amplitude-modulators and the synchronous demodulator, in order to be able to adjust phase until the phase errors disappear. However, making such phase adjustments is complicated by the unavailability of suitable vectorscopes for processing the carrier frequencies used in this mixing technique. Also, even if a successful phase adjustment is achieved, it is necessary thereafter to continuously monitor for renewed development of such phase errors and then to adjust phase once again until such phase errors are again eliminated.